The present invention relates to a single-lens reflex electronic still camera including an image pickup device for photoelectrically converting the image of an object, a memory device for storing a still-image signal produced by the image pickup device, and an optical viewfinder.
Such an electronic still camera employs an image pickup device such as a solid-state image pickup device for producing electric signals representative of the brightness of an object image and a magnetic disk or semiconductor memory for storing the generated electric signals. Many such electronic cameras have been proposed as a substitute for the film conventional cameras because of the advantages that it is not necessary to use film requiring chemical treatment and the images can quickly be reproduced on a television receiver.
The electronic cameras can incorporate many types of viewfinder systems found in the prior art film cameras, and also can use an electronic viewfinder employed in an ordinary video camera utilizing the image pickup device of the electronic camera. Of these viewfinder systems, the so-called single-lens reflex viewfinder is functionally the best since the viewfinder area is as big as the image area, and interchangeable lenses can be employed.
Two types of single-lens reflex viewfinders are known. One single-lens reflex viewfinder type has a half-silvered mirror or beam splitter disposed in the image light path for reflecting part of the light into the optical viewfinder system. The other viewfinder construction uses a so-called quick-return mirror for guiding the image light selectively into the optical viewfinder system and the optical photographing system. The quick-return mirror viewfinder system is considered to be advantageous over the half-silvered mirror viewfinder system in that, although it has moving parts, the image visible through the viewfinder is bright, and there is no reduction in the quantity of light reaching the photographing system.
For incorporating the single-lens reflex viewfinder, particularly such a viewfinder of the quick-return mirror type, into the electronic camera, it is required that the drive unit for the quick-return mirror be electrically connected to the photographing system, resulting in the need of a novel mechanism different from those of conventional cameras using film.
Prior electronic cameras employing the single-lens reflex viewfinder with the half-silvered mirror have suffered from drawbacks in that the image on the viewfinder is relatively dark and the quantity of light entering the photographing system is reduced, resulting in a failure to provide a good quality still image of a dark object. The reduction in the amount of image light available for photographing is particularly disadvantageous since presently available solid-state image pickup devices have a lower sensitivity than film.
Another problem relates to the generation of signals for reading electric charges from the image pickup device. A prior video camera, for example, uses an image pickup device for photoelectrically converting the optical image of an object, the image pickup device being driven by a vertical synchronizing signal composed of cyclic pulses each having a period of one field (1/60 sec. in the NTSC system). With such pulses being used both for discharging unwanted stored electric charges prior to photographing and for effecting the reading of electric charges serving as still-image signals, an appreciable period of time is required for an exposure to be effected after a release button has been depressed and for electric charges to be read out after exposure has been completed. This results in an appreciable undesired dark current which reduces the quality of an image. The problem of delayed photographing operation cannot be avoided. For the above reasons, it is not advisable to apply these video camera techniques to an electronic still camera.
The mirror and shutter could be electrically driven by utilizing a motor drive unit from prior cameras. However, the conventional motor drive unit has a takeup mechanism of a complicated construction for winding film with a large force and over a relatively long interval of time. The electric circuit for such a motor drive unit is also complex as it requires a switch mechanism for indicating the completion of a film winding operation. Therefore, the prior motor drive unit arrangement cannot be directly incorporated into an electronic still camera.